The present invention concerns improved hard-soft ionomeric resin mixtures (or blends) which are particularly well suited for the formulation of the cover composition of a golf ball.
More specifically, the present invention relates to novel golf ball cover compositions comprising blends of hard ionomer resins with recently developed soft ionomer resins made from acrylic acid based polymers. The new compositions of the present invention, when utilized for golf ball construction, particularly the construction of two piece golf balls, produce golf balls exhibiting enhanced travel distance without sacrificing the properties of playability and/or durability.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E.I. DuPont deNemours & Company under the trademark "Surlyn.RTM." and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks "Escor.RTM." and the tradename "Iotek", have become the materials of choice for the construction of golf ball covers over the traditional "balata" (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability necessary for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc., for golf ball cover construction over balata. However, the advantages gained in increased durability have been offset to some degree by the decreases produced in playability. This is because although the ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the "hard" ionomeric resins, but also the playability (i.e. "spin") characteristics previously associated with the "soft" balata covers, properties which are still desired by the more skilled golfer.
In various attempts to produce such an ideal golf ball, the golfing industry has blended the hard ionomeric resins with a number of softer polymeric materials, such as softer polyurethanes. However, the blends of the hard ionomer resins with the softer polymeric materials have generally been dissatisfactory in that these balls exhibit numerous processing problems. In addition, the balls produced by such a combination are usually short on distance.
In addition, various "hard-soft ionomeric blends", i.e. mixtures of ionomer resins which are significantly different in hardness and/or flexural modulus, have been attempted. However, until the development of the specific blend combination set forth in U.S. Pat. No. 4,884,814, these balls were not particularly commercially viable. In this regard, although the balls produced using the hard-soft ionomer blends exhibited enhanced playability characteristics, they lacked the durability needed for continuous play.
U.S. Pat. No. 4,884,814, the present inventors previous patent, is directed to the finding that if various "hard" Surlyn.RTM. methacrylic acid based ionomer resins (i.e. those Surlyn.RTM. resins having a hardness of about 60 to 66 on the Shore D scale as measured in accordance with ASTM method D-2240) were blended with a number of specific "soft" Surlyn.RTM. methacrylic acid based ionomer resins (i.e. those Surlyn.RTM. resins having a hardness from about 25 to 40 as measured on the Shore D scale) that a golf ball cover composition could be produced that is not only softer than the prior art hard ionomer covers but also exhibits a sufficent degree of durability for repetitive play.
As a result, a golf ball covered in accordance with the "hard-soft" methylacrylic acid based ionomer blends of the '814 patent, exhibits properties of enhanced playability (i.e. softness and spin) and durability. However, notwithstanding the above, some sacrifice in carrying distance is also exhibited in comparison with the balls produced utilizing the hard Surlyn.RTM. resins.
The present invention is directed to new golf ball cover compositions which exhibit properties of enhanced carrying distance (i.e. possess higher coefficient of restitution values) than the hard-soft ionomer blends set forth in U.S. Pat. No. 4,884,814, without sacrificing characteristics such as playability (i.e. softness and spin) and/or durability. It has been found that these properties can be produced using improved hard-soft ionomer blends containing recently developed acrylic acid based soft ionomer resins.